fix: preserve original value when Content-Type is unrecognized

[Nicknamess96]

When res.set('Content-Type', value) is called with a value that mime.contentType() cannot resolve (returns false), the Content-Type header is currently set to the literal string "false".

This happens because mime.contentType(value) returns false for unrecognized types, and that false value is passed to setHeader(), which coerces it to the string "false".

The fix preserves the original user-supplied value when mime.contentType() returns false, so res.set('Content-Type', 'some-custom-type') will set the header to "some-custom-type" instead of "false".

This is consistent with how res.type() handles unknown types (it falls back to "application/octet-stream" rather than propagating false).

Reproduction:

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.set('Content-Type', 'some-custom-type');
  res.send('hello');
});
// Response has Content-Type: false

Changes:

• lib/response.js: Only assign the result of mime.contentType() when it returns a truthy value
• test/res.set.js: Add 5 test cases covering known types, shorthand types, unknown types, the false regression, and full MIME types with slashes

Fixes #7034

[krzysdz]

Duplicate of #7035